1. Field of The Invention
The present invention broadly relates to an automotive automatic transmission, particularly to a controlling hydraulic circuit suitable for use in a multi-speed automatic transmission which is comprised of a main transmission unit and a sub-transmission unit. More particularly, the invention is drawn to a drain control arrangement for hydraulic servomotors which actuate various frictional engagement elements incorporated in the controlling hydraulic circuit.
2. Description of the Related Art
Generally, an automatic transmission has a torque converter and a planetary speed changing gear mechanism. The planetary speed changing gear mechanism is comprised mainly of an overdrive (O/D) planetary gear unit, a front planetary gear unit and a rear planetary gear unit. The planetary speed changing gear mechanism performs, by engaging or retaining a predetermined element by a clutch or a brake (frictional engagement elements) under the control of two solenoid valves and three shift valves, a speed change over four forward speeds and one reverse speed.
The frictional engagement elements are usually operated by hydraulic servomotors which are supplied with hydraulic pressure or which are drained in accordance with the states of the shift valves. Usually, each of such hydraulic servomotors is associated with an accummulator connected in parallel therewith, so that the servo pressure of the servomotor, i.e. the engaging/disengaging characteristic of the frictional engagement element, is determined by the hydraulic pressure characteristics of the accummulator. Particularly, the draining characteristics of the accummulator directly affect the releasing characteristics of the servomotor.
Hitherto, however, the hydraulic pressure characteristics of the accummulator have been fixed, so that the releasing characteristics of the servomotor and, hence, of the frictional engagement element could not be controlled in accordance with factors such as the vehicle speed, timing of engagement of another frictional engagement element, and so forth.
In recent years, multi-speed automatic transmissions have been proposed in which a plurality of speeds are obtained by complicated control of engagement and disengagement of various clutches and brakes. Such automatic transmissions encounter a problem in that a large impact tends to be generated when the speed is changed, due to the draining characteristics of the accummulators having the aforementioned predetermined hydraulic pressure characteristics.
This problem will be described in more detail with reference to a multi-speed automatic transmission proposed in Japanese Patent Laid-Open No. 37140/1982, in which six speeds are attained by the combination of a sub-transmission unit which has two speeds, i.e. overdrive speed and direct-coupling speed, and a main transmission unit which can select one of three speed gears, i.e. a 1st speed gear, a 2nd speed gear and a 3rd speed gear. In some cases, this automatic transmission is required to effect simultaneously both an upshift in one of the main and sub-transmission units and a downshift in the other. For instance, upshift of the automatic transmission as a whole from second to third speed is attained by effecting simultaneously both a downshift of the sub-transmission unit from the overdrive speed to the direct-coupling speed and an upshift of the main transmission unit from 1st to 2nd speed. However, the conventional controller which makes use of shift valves and accummulators is not constructed to simultaneously effect both shifting operations. Therefore, practically, either one of the shifting operations lags behind the other. For instance, when a downshift operation is completed first, the automatic transmission as a whole is first shifted down to the first speed to reduce speed greatly and is then upshifted to the third speed to drastically increase speed. Conversely, when the upshift is completed first, the automatic transmission as a whole is first upshifted to the fourth speed to drastically increase in speed and is then downshifted to reduce speed. In both cases, the engine is abruptly accelerated or decelerated and a large impact is generated in the automatic transmission, thereby impairing the driveability of the automobile.